1. Field of the Invention
The present invention relates generally to the measurement of cardiac output and, more particularly, to thermodilution cardiac output measurement techniques.
2. Description of the Related Art
Cardiac output, conventionally defined as the amount of blood pumped by the heart over several cardiac cycles, is a fundamental performance measurement for assessing the condition of the heart. For example, cardiac output is routinely measured as part of diagnostic cardiography and patient monitoring.
Physicians' concern with cardiac output generally relates to the output of the left heart, which supplies the body with oxygenated blood. Since the output of the two halves of the heart are equal, cardiac output can be measured on either side of the heart. However, left heart measurements are much more difficult and dangerous than right heart measurements. In a left heart measurement, the catheter must be threaded into the heart via an artery, against the direction of blood flow, into the left ventricle and finally into the left atrium. Bleeding, infection, and other complications are common risks. By contrast, the measurement of right heart output is considerably safer and easier to perform, particularly because the catheter is threaded through the venous system. Consequently, cardiac output is most often determined by performing right heart measurements.
The technique currently accepted by the clinical community to measure cardiac output is known as the "bolus thermodilution" measurement technique. A measurement catheter is threaded through a vein in either the thigh or the shoulder until the tip of the catheter, containing a thermistor (an electrical temperature sensor), is located in the pulmonary artery, and a fluid ejection port of the catheter is located in the right atrium of the heart. To make a measurement, the administering clinician injects a small quantity (approximately 10 cc) of cold saline solution through the catheter into the right atrium via the port. The cold saline solution mixes with the blood in the right atrium and ventricle, and the mixture is ejected by the heart into the pulmonary artery. The mixing of cold solution with the blood cools the blood temperature by 0.5 to 1 degree Celsius, an effect that dissipates over the next several heartbeats, depending on the flow rate. The thermistor measures the temperature of the mixture as the cooled blood is ejected from the heart over the next few heartbeats. An energy balance equation is then used to calculate the average flow from the temperature-time data.
While conceptually simple, this technique has several drawbacks. Due to various sources of measurement noise, any one cardiac output measurement has a high level of uncertainty. As a result, from 3 to 6 repetitions are required to establish confidence in the reading. The total fluid load resulting from these repetitions can be harmful to the patient. Accordingly, the measurements are intermittently performed, with the approximate maximum being one set of 3-6 repetitive measurements per hour. This periodic performance of the test prevents this technique from providing the clinician with continuous cardiac output measurements.
Furthermore, the measurements are labor-intensive: an operator's intervention is required to make each fluid injection. As a result, the measurements are operator-dependent, and vary according to the "injection technique" of the administering clinician. Finally, there are additional sources of measurement error, including errors due to the warming of the saline in the cooled container and in the catheter prior to and during injection. Furthermore, natural variations in the nominal blood temperature, typically approximately 10 millidegrees, introduce additional measurement errors.
Techniques proposed to overcome these shortcomings have generally consisted of replacing the injection of chilled solution with a small heater on the surface of the catheter to cause a localized temperature change in the blood. An example of this approach is described in U.S. Pat. No. 4,576,182 to Normann (hereinafter "Normann"). In Normann, a catheter with an electrically-resistive heating element wound around its outer surface is heated by the discharge of a capacitor through the heating element. Although this technique permits automatic and rapid measurements while eliminating fluid overload and operator-induced variation problems associated with the use of chilled saline solution, there are a number of drawbacks which make this approach impractical.
Specifically, although the blood cells sustain the cooling caused by the injection of saline with no damage, they are very susceptible to damage caused by increases in temperature. Heat enters the blood by conduction from the surface of the catheter to the relatively cool flowing blood. The maximum temperature in the blood thus occurs at the catheter/blood interface. To deliver sufficient heat to the blood to obtain a reliable temperature signal may require a relatively high temperature at the interface, such as to damage blood cells adjacent to the heater. This problem is exacerbated when the resistive heating element directly contacts the blood.
In fact, the Underwriter's Laboratory standard for Medical Electronics, UL544, limits contact temperature of medical devices to 41 degrees Celsius. Similar limits are imposed by IEC standards in the international community. Heat transfer considerations therefore limit the power delivered to the blood using standard size catheters to less than 10 watts. This limitation on the power levels of the resistive heaters severely reduces the signal-to-noise ratio for this technique, which in turn reduces its accuracy. The reduction in signal strength is best illustrated by comparing this power to the saline injection technique, wherein a 10 cc, 0 degree Celsius saline injection over 2 seconds is equivalent to the removal of 750 watts of power from the blood.
Another prior approach, described in U.S. Pat. No. 4,621,929 to Phillips (hereinafter "Phillips"), uses anemometric techniques to heat and measure fluid flow. In Phillips, various external energy sources are used to excite and heat an absorber within the catheter to serve as the heat source within the fluid. This technique, however, also relies on conduction of this energy into the fluid, and thus has limitations on the total power similar to the resistive thermal heater approach described above.
Phillips also describes the use of laser energy to directly heat the blood in conjunction with various methods of determining flow rates. In such a method constant-power heating is used, with the flow rate determined by measuring the temperature increment downstream. In another Phillips method, the laser power is adjusted to maintain a constant temperature increment downstream, in which case the flow is determined by measuring the power required to maintain the constant temperature. These latter techniques suffer from two conflicting problems.
First, the analysis method implemented in Phillips requires that steady state thermal conditions be achieved prior to taking readings. This in turn requires that the blood be heated for a sufficiently long period of time. However, the continuous heating of blood to perform these measurements can add significantly to normal basal energy production and can cause the patient to overheat. As a result, Phillips must restrict the power to such a degree that the benefit of directly heating the blood is lost.
Second, the technique itself will cause an increase in the baseline blood temperature, which will necessitate numerous cessations in the heating to monitor this temperature drift. In order to monitor the baseline temperature of the blood, Phillips cycles the heater power, waiting for the temperature increments to decay, and then measuring the temperature to establish the new baseline. Once it is measured, power is reapplied to the heater. Thus, this approach necessitates numerous cessations in the heating to monitor this temperature drift, which prevents the achievement of the steady state temperature levels necessary to make temperature measurements.
Another conventional approach, described in U.S. Pat. No. 4,785,823 to Eggars et al. (hereinafter "Eggars"), applies a high-frequency electric potential difference to electrodes in the catheter to generate a current though the blood to create a bolus of blood at an elevated temperature. There are significant drawbacks to this approach, however.
First, the heating is directly dependent on the current density, which is significantly higher near the electrodes than at a distance away from the electrodes. Thus, the resulting heating field is not uniform, with the blood immediately adjacent to the electrodes reaching a higher temperature than the majority of the blood located at a distance from the electrodes. In addition to the nonuniform heating of the blood, this also results in damage to the blood cells adjacent to the electrodes or limits the power to such a degree that signal reliability is reduced.
Another problem with the Eggars approach is that the current field for reasonable resistance values spreads out fairly far into the blood, resulting in a non-containable energy field. To obtain a desirable energy transfer, electrodes having a resistance of approximately 50 ohms are preferred. This requires a significant electrode spacing which will likely result in the current entering the heart wall. The inevitable loss of energy into the heart wall violates the energy balance on which this measurement is based, and thus renders the measurement inaccurate.
Another problem with the Eggars approach is that the resistance between the electrodes is a function of the distance between them. The resistance in turn, affects the power transferred to the blood. There will be relative movement of the electrodes due to the bending of the catheter during use, causing the spacing between the electrodes to change. This will alter the resistance between the electrodes, thereby altering the power that will be delivered to the blood. This leads to further measurement inaccuracies.
What is needed, therefore, is a means for measuring cardiac output that enables essentially continuous measurement with a sufficiently high energy transfer while simultaneously minimizing the heat load experienced by the patient. Such a measurement system must be capable of responding to natural variations in the baseline blood temperature and allow flexible heating and analysis techniques to minimize the effect of thermal noise in the blood on the measurement of the flow rate.